The smaller of twin panda cubs born over the weekend to giant panda Mei Xiang died on Wednesday afternoon, Washington's National Zoo said.

The death of the cub, whose up-and-down weight since the birth on Saturday had raised concerns among the zookeepers, occurred shortly after 2 p.m. on Wednesday.

The twins' birth captured international attention as giant pandas are among the world's most endangered species.

"We are sad to report that the smaller of the two panda cubs has died," @NationalZoo posted on Twitter.

"The larger cub appears to be strong, robust, behaving normally and is with mother Mei Xiang," the zoo said in a press release.

A day before the cub died, zoo keepers noted that Mei Xiang, a star tourist draw in the U.S. capital, was focusing her care on the larger twin. Both cubs were still fur-less and about the size of a stick of butter.

The zoo staff was caring for the smaller cub and was trying to swap the cubs in Mei Xiang's possession every four hours, it said.

Mei Xiang was artificially inseminated with sperm from Hui Hui, a panda in China, and from the National Zoo's Tian Tian.

Zoo officials have said that they did not yet know which insemination was successful, and that it was possible the twins had different fathers.

Giant pandas, native to China, have a very low reproductive rate, especially in captivity. There are about 300 giant pandas in captivity and roughly 1,600 in the wild.